Many industrial processes employ catalysts comprising a catalytically active material supported on a shaped body or bound in a shaped body. Examples of such catalysts include supported metals, supported metal complexes, supported organometallics, bound zeolites and bound zeolite-type materials. The shaped bodies used in such catalysts are typically made of refractory inorganic oxides by forming a mixture of one or several sources of the inorganic oxide in a suitable vehicle, such as water and/or an organic solvent. The mixture is then formed into various shapes, generally by extrusion, and subsequently dried and calcined to produce the final shaped body.
In order to be used as catalyst carriers, these bodies must have appropriate surface properties, sizes, shapes and porosities to carry the desired amounts of active materials. The shaped bodies must also be strong enough to enable catalyst handling, especially during reactor loading and unloading, and to sustain the conditions experienced during the intended catalytic use. They must also have appropriate porosities and shapes to avoid high pressure drops across the reactor and allow the desired catalytic reactions to take place.
The choice of catalyst carrier will depend on various factors, such as, for example, the intended catalytic use, the type of catalytic material used, the required catalyst strength and the required diffusivity across catalyst particles. The most commonly used carrier materials have included alumina and mixtures of the same with clays for the reason that these materials are very easy to extrude and result in the production of an extrudate having desirable physical strength as well as controllable diffusion properties.
For certain catalytic processes, such as the conversion of methanol and other oxygenates to olefins, it is desirable to use silica as the catalyst carrier. However, forming silica bound catalysts by extrusion has in the past proved difficult since processing silica-rich mixtures in conventional extruding equipment tends to give products with poor mechanical strength. This is particularly problematic in the case of crystalline silicate catalysts, such as zeolitic catalysts, where the catalytic material itself is silica-rich (that is contains more than 50 wt % silica).
One solution to the problem of producing silica extrudates with improved strength is proposed in U.S. Pat. No. 4,582,815. This patent discloses a method for extruding silica-rich solids which comprises mixing the silica-rich solids with water and an alkali metal base or basic salt followed by mulling and extruding wherein the amount of water added is sufficient to have a total solids content of from 25 to 75 weight percent, the alkali metal compound being added in an amount of from 0.25 to 10 weight percent on a dry basis based on the total solids calculated as sodium hydroxide equivalent, and drying and calcining the extrudates.
The extrusion method described in U.S. Pat. No. 4,582,815 has proved very effective in producing silica-bound catalysts with crush strength approaching that of extrudates made from alumina. However, when practiced on a commercial scale, it is found that the extrusion is accompanied by significant loss of solids due to the phenomenon known as “feathering”. This means that the extrudate, instead of having a smooth outer surface, exhibits microscopic cracks in its surface where small flakes or “feathers” of the extrudate have separated from the surface. This not only causes loss of valuable catalyst material but also tends to impair the physical strength of the extrudate.
It has now been found that the problem of “feathering” during extrusion of silica rich mixtures can be reduced or avoided by employing potassium hydroxide instead of sodium hydroxide in the process of U.S. Pat. No. 4,582,815. Not only is the loss of solids as fines during the extrusion process reduced by the use of a potassium-derived alkali metal base or basic salt, but also the crush strength of the final calcined product is improved.
U.S. Pat. Nos. 6,576,120 and 6,709,570 disclose a method for preparing catalysts comprising ZSM-5 and silica which comprises: (a) preparing an extrudable mass by first mixing ZSM-5, an amorphous precipitated silica and an acid colloidal silica into a first homogeneous mixture having a pH below 7 and subsequently adding ammonia to the first homogeneous mixture such that the pH of the resulting second mixture has a value of above 8, (b) extruding the extrudable mass resulting from step (a), (c) drying the extrudate resulting from step (b); and, (d) calcining the dried extrudate resulting from step (c). However, not only do these patents fail to address the problem of “feathering”, they require the use of an expensive acid colloidal silica and the provision of a scrubber or other gas treatment device to remove the ammonia vapors that are necessarily released in the drying and calcining steps.
International Publication Number WO 2006/026067-A1 describes a method for the manufacture of a structured body, which process comprises (a) preparing a batch composition free of organic solvent comprising (i) at least one particulate inorganic material, (ii) at least one particulate silicone resin of average particle size 700 microns or less, and (iii) water, and (b) forming the batch composition into a structured body. While this method provides shaped bodies with excellent properties, it requires the use of silicone resins that can be expensive if used on a very large scale.